collatz.py

lookup = {}
def collatz(start, iteration):
    if start == 1:
        return iteration + 1
    if start in lookup:
        return collatz(lookup[start], iteration + 1)
    if start % 2 == 0:
        lookup[start] = start / 2
        return collatz(lookup[start], iteration + 1)
    else:
        lookup[start] = start * 3 + 1
        return collatz(lookup[start], iteration + 1)

class Largest(object):
    def __init__(self, iterations=0, number=0):
        self.number = number
        self.iterations = iterations

    def update(self, number, iterations):
        self.number = number
        self.iterations= iterations

    def __str__(self):
        return "{} has {} iterations".format(self.number, self.iterations)

largest = Largest()
for i in xrange(1, 10**6):
    num_iterations = collatz(i, 0)
    if num_iterations > largest.iterations:
        largest.update(i, num_iterations)

print(largest)